1. Field of the Invention
This invention relates generally to rotary tables. More particularly, the present invention relates to a rotary indexable table having a cycloidal drive gear.
2. Background Information
Rotary tables, such as rotary indexable tables, are well-known for the accurate positioning of work pieces at work stations for automated operations. Rotary indexable tables typically have a table and an indexer assembly that rotates the table through a predetermined angle for positioning work pieces for sequential automated operations.
Rotary indexable tables have been successfully employed in the field of automated assembly for work stations including pick and place devices, feeder bowls, visual inspections, label applicators, robot arms, adhesive applicators, laser machining and other automated assembly processes. Rotary indexable tables are further well-known in the fields of machining for the accurate positioning of work pieces to receive drilling, boring, tapping, CNC machining, facing, grinding, and other types of machining processes. Other uses for rotary indexable tables include the accurate positioning of work pieces for coating, sterilizing, cleaning, testing and calibrating.
As described in U.S. Pat. No. 5,950,503, which is incorporated herein by reference, rotary indexable tables have also been used in the decorating field for screen printing, hot stamping, pad printing, ink jet printing, impact marking, laser marking, spray painting and other decorative processes. For example, rotary indexable tables are currently employed for multi-color screen printing onto work pieces such as CD's, credit cards, key fobs, etc. Typically, a rotary indexable table supports multiple, equidistantly positioned fixtures. The fixtures receive and support the work pieces during the printing operations. At a first work station, a work piece is automatically positioned onto the fixture. The table then rotates through a precise angle or distance to position the work piece under a first screen printing apparatus. After the printing is completed, the table rotates through the same angle again to position the work piece for receiving a second overlaying screen print image. The indexing process continues until the work piece has received all the required layers of screen printing and is removed from the fixture at a final work station.
With the need for very precise machining and close tolerances in manufacture, rotary indexable tables have had to be much more precise and provide more through-put in order for the industry to remain competitive. Rotary indexable tables, for example, may be required to move through a complex set of rotary profiles such as continuous rotation, indexing with a dwell time, oscillation, variable speed or reverse direction. It would be advantageous to have an assembly capable of all these motions while maintaining precision. In addition, with the advent of robotics these assemblies are required to place a work piece at various work angles relative to the work station to provide access from automated operational equipment.
Typically, prior art rotary indexable tables, also known as turntables, are centrally driven and work is performed at the periphery of the table. Alternately, when tables are driven on their outside diameter, the drive mechanism tends to be outside the periphery of the table and thus impedes use of the assembly in various angles and in operations where space is at a premium. Additionally, this configuration interferes with operations to be performed with large overhanging work pieces which was discussed in U.S. Pat. No. 5,950,503 addressing the precision of the drive system for an open center turntable, and which is incorporated herein by reference. Because of the preciseness of the operations to be performed at a particular work station, the indexing and rotational positioning of such tables has become hypercritical.
Additional attempts have been made to improve the operation of turntables. For example, U.S. Pat. No. 2,965,208 discloses a turntable system utilizing a central shaft operably connected to its drive to rotate the table. The table also supports a plurality of rotatable spindles separately driven to accommodate actions requiring rotation of the work piece at an indexed position of the table. U.S. Pat. No. 5,170,678 discloses an index table which utilizes a table top having an open center and ring gear/pinion gear drive system located beneath the rim of the table top. The table top is supported on a fluid (air or oil) cushion during rotation. After the position is indexed, the cushion is removed and the table top lowered by gravity into place on a fixed support. While suited for its intended purpose, this assembly still suffers from the limitations herein discussed and is not readily adaptable for operation in a variety of orientations. A commercial assembly marketed by Camco utilizes a shaft-driven cam to drive an index table wheel having a center hole and supported by a bearing system located beneath it. This system is also limited in its operational versatility, is still weighty, and does not minimize energy requirements.
It is an object of the present invention to improve the precision of the drive of a rotary table, such as a rotary indexable table. One aspect of the present invention is to look toward distinct gearing arrangements to improve the drive of a rotary indexable table. For example, U.S. Pat. No. 4,879,920 pertains to antibacklash gears that mesh, including rack and pinion gears and, in particular, to gear tooth geometries that both minimize sliding friction and limit backlash between the gears.
Antibacklash rack and pinion gear sets are disclosed in U.S. Pat. Nos. 2,548,603, 4,066,356 and 4,228,696. Antibacklash gears utilizing displaced parallel gears meshing with common pinions are disclosed in U.S. Pat. Nos. 2,966,806, 3,035,454 and 3,037,396. U.S. Pat. No. 3,122,938 discloses an elastic web between a rigid hub and a rigid rim and teeth to preload the gear set and reduce backlash. U.S. Pat. No. 3,213,704 discloses light duty gear sets, wherein the gears are positioned to cause a resilient distortion and radial flexing to provide the zero backlash effect. U.S. Pat. No. 4,305,307 discloses a separate driven pinion connected to a torsion spring shaft that is adjusted to twist and thereby load the antibacklash driven gear.
Numerous patents have issued disclosing a wide variety of specific geometries to reduce noise and wear characteristics of gear sets. U.S. Pat. No. 1,828,114 discloses mating involute spur gears with sufficient teeth so formed that at all times at least two teeth of each mating gear mesh with at least two teeth on the other gear, thereby preventing overloading of a single tooth on each gear. U.S. Pat. No. 4,140,026 discloses an improvement to conformal gearing by providing flexure to the concave teeth to accommodate the changing geometry as the teeth wear or the center to center distance between the gears increases. Circular arc gear teeth are disclosed in the pinion and internal gear sets of U.S. Pat. No. 4,270,401, wherein a specific tooth geometry is specified.
Specific gear tooth geometries for helical gears are disclosed in U.S. Pat. Nos. 4,292,857 and 4,294,132, both directed to teeth of changing thickness and changing pitch diameter in traversing from the left to the right side of each gear. The specific geometry is directed to controlling backlash between the meshing gears. Specific tooth geometries for rack and pinion backlash elimination is disclosed in U.S. Pat. No. 3,310,990. In this disclosure, the involute gear and rack have modified tooth profiles which define gear contact depending upon the position of the rack relative to the pinion.
Gears that mesh with pins and rollers are sometimes referred to as sprockets, in particular, when the pins or rollers are attached to chains. U.S. Pat. Nos. 4,036,071 and 4,116,081 disclose sprocket and chain combinations wherein the teeth on the sprockets engage rotatable bushings on the chains. In the latter patent, the chain is a part of a crawler tack for a tracked vehicle. The sprocket teeth and rollers undergo severe operating conditions and, therefore, the tooth geometry is selected to minimize stress and wear, and in the case of the tracked vehicles, to also accommodate the buildup of foreign matter between the teeth of the sprocket. In U.S. Pat. No. 4,487,088, the sprocket teeth are relatively widely separated, with the bushings of the chain riding against the roots of the sprocket teeth under high load. These patents disclose relatively large root areas between the sprocket teeth. Conversely, U.S. Pat. No. 3,709,055 discloses teeth and root areas of circular arcs meshing to provide the lobes of a gear pump. U.S. Pat. No. 4,155,599 discloses a driving pinion having epicycloidal gear teeth meshing with cylindrical pins of a rack. The gear teeth are purposely formed with a slender profile to penetrate into the spaces between the cylindrical pins forming the rack. The profile is specifically designed to accommodate differences in pitch distance between the cylindrical pins where racks are joined together lengthwise.
Cycloidal gearing, in general, is the predecessor of involute gearing. Cycloidal gearing has been broadly used in mechanical watch mechanisms. Pin gearing has been a particular case or subset of general cycloidal gearing. Historically, the main advantage for using pin gearing is the possibility of avoiding the generation of the pinion teeth since the pinion is designed as an assembly of cylinders placed between two disks. Further, pin gearing has been examined for its antibacklash gear property.
As stated above, it is an object of the present invention to improve the precision of the drive of a rotary indexable table. It is a further object of the present invention to provide a reliable easily manufactured rotary indexable table and drive associated therefor.